There are increasing occasions for ordinary households to directly receive a plurality of satellite broadcastings in recent years, as satellite broadcastings have come to a stage of being wide spread and communication satellite broadcastings in digital signal have since started using commercial communication satellites. With this trend, there is an increasing demand for reduction in both size and cost of antennas for receiving the broadcastings. In case of the communication satellite broadcastings, receiving antennas often incorporate therein a low-noise down-converter having a polarization selecting function since they are multi-channel broadcastings by utilizing electromagnetic waves of different polarizations (e.g. horizontally polarized waves and vertically polarized waves) of same frequency for a reason of efficient use of the frequencies.
A microwave mixing circuitry and an intermediate frequency amplifier having polarization selecting function of the prior art are described hereinafter by referring to FIG. 5.
Both horizontally and vertically polarized microwave signals (e.g. signals in a frequency band of 12 GHz) input to microwave signal input terminals 101 and 102 are converted into intermediate frequency signals of 1 GHz in Schottky barrier diodes (hereinafter referred to as "SBD") 148 and 149 for frequency conversion connected to microwave strip transmission lines (hereinafter referred to as "MSL") 106 and 107 by being mixed respectively with local oscillation signals (e.g. signals of 11.2 GHz) supplied from a local oscillator 103 via band-pass filters (hereinafter referred to as "BPF") 104 and 105.
Here, a frequency conversion loss in the SBD's 148 and 149 increases in case that an output of the local oscillation signal supplied by the local oscillator 103 is small. The SBD's 148 and 149 are provided with bias currents in the forward direction in order to prevent an increase of the frequency conversion loss. These bias currents are supplied from bias terminals 110 and 111 through connections with anodes of the SBD's 148 and 149.
The intermediate frequency signals that have passed through lowpass filters (hereinafter referred to as "LPF") 112 and 113, which allow the intermediate frequency signals to pass through, further pass through PIN diodes 138 and 139 after having amplified by intermediate frequency amplifiers 134, 135, 136 and 137.
Current supply terminals of the intermediate frequency amplifiers 134 and 135 and an anode of the PIN diode 138 are connected with a collector of a transistor 142 through an input signal selection control terminal (hereinafter referred to as "polarization selection control terminal") 146 for selecting between input signals of horizontally polarized waves and vertically polarized waves.
Likewise, current supply terminals of the intermediate frequency amplifiers 136 and 137 and an anode of the PIN diode 139 are connected with an emitter of a transistor 143 through a polarization selection control terminal 147.
The microwave mixing circuitry is supplied with a polarization selection signal (e.g. a D.C. voltage of either 11 V or 15 V) from the outside through an intermediate frequency signal output terminal 145.
A comparator 144 outputs a D.C. signal of different values in response to the polarization selection signal.
In the event the polarization selection signal is 11 V, the transistor 142 turns on and the transistor 143 turns off simultaneously. As a consequence, an electrical potential of the polarization selection control terminal 146 increases whereas that of the control terminal 147 decreases.
On the other hand, in the event the polarization selection signal is 15 V, the transistor 143 turns on and the transistor 142 turns off simultaneously. Hence, the electrical potential of the control terminal 147 increases whereas that of the control terminal 146 decreases.
In case the polarization selection signal is 11 V, the intermediate frequency amplifiers 134 and 135 and the PIN diode 138 turns on and the intermediate frequency amplifiers 136 and 137 and the PIN diode 139 turns off, so that intermediate frequency amplifiers 140 and 141 are supplied with an intermediate frequency signal corresponding to the horizontally polarized microwave signal input through the microwave signal input terminal 101, and the signal is output from the intermediate frequency signal output terminal 145 after being amplified to a desired magnitude.
In the same way, if the polarization selection signal is 15 V, the intermediate frequency amplifiers 134 and 135 and the PIN diode 138 turns off and the intermediate frequency amplifiers 136 and 137 and the PIN diode 139 turns on, so that intermediate frequency amplifiers 140 and 141 are supplied with an intermediate frequency signal corresponding to the vertically polarized microwave signal input through the microwave signal input terminal 102, and the signal is output from the intermediate frequency signal output terminal 145 after being amplified to a desired magnitude.
With this structure of the prior art, however, it is quite difficult to reduce the size as is also inimical to the cost reduction because it requires the intermediate frequency amplifiers 134, 135, 136 and 137 and the PIN diodes 138 and 139 responsive to the microwave signal input terminals 101 and 102 to which two different polarized waves are input.